The present invention relates to digital communications systems. More specifically, the present invention relates to baud clock synchronization between base stations and subscriber units of time division multiple access digital communications systems.
In many digital radio communications systems, communication between base stations and subscriber units is effected in both the frequency domain (i.e., utilizing multiple frequency channels) and the time domain (i.e., utilizing multiple time slots per frequency channel). It is desirable that the resultant composite spectrum be used efficiently.
In the frequency domain, the efficient use of the available spectrum depends in a large part upon the methodology used to maintain spectral occupancy. Modulation, through nonlinear amplification, or through crossover, intermodulation, or other distortions, tends to cause a spreading of the bandwidth required of a given signal. This creates a need for guard bands, i.e., unassigned frequencies between allocated channels, inversely proportional to the spectral occupancy. The greater the spectral occupancy (i.e., the less the signal occupies unallocated frequencies), the lesser the guard band allocation required in compensation, and the more efficient the frequency allocation scheme.
A common method of improving spectral occupancy in digital communications systems is to utilize a pulse shaper to effectively transfer signal spread from the frequency domain to the time domain. By using filters that spread the energy from each signal pulse over several baud intervals, spectral occupancy of the digital signal is significantly maximized. This increase in spectral occupancy allows the achievement of a more efficient frequency channel allocation scheme.
Unfortunately, spreading a given pulse over multiple baud intervals creates an inter-symbol interference (ISI) problem. ISI exists when the energy from one symbol pulse interferes with the energy from another symbol pulse, masking or otherwise distorting portions of the data stream.
The use of a bandwidth-efficient pulse shaper, such as a square-root Nyquist (SRN) pulse shaper, can minimize the effects of ISI. With an SRN pulse shaper, each individual pulse can extend up to ten or more baud intervals before and after the peak baud interval. When a given pulse is passed through an SRN pulse shaper, it exhibits maximum energy at only one instant and zero energy at all other instants displaced before and after the peak instant by integral baud intervals. In a typical data stream where pulses are displaced by integral baud intervals, when the energy of a given pulse is at its peak, the energy of every other pulse is at zero, thus effectively controlling ISI.
The use of an SRN pulse shaper spreads the energy of a given pulse over a significant number of baud intervals. This creates significant ISI between adjacent pulses that are not separated by an integral number of baud intervals, such as signals from different subscriber units having adjacent time-slot allocations in the time domain. This time domain spreading creates a need for guard slots, i.e., unassigned time slots between allocated time slots. In other words, by exchanging a signal spread in frequency for a signal spread in time, the use of an SRN pulse shaper has exchanged an allocation inefficiency in the frequency domain for one in the time domain.
A like situation may also occur in an FDMA system where a user is assigned a frequency channel immediately after a previous user has vacated that channel. Care must be taken to prevent interference between the trailing pulses of the previous user and the leading pulses of the new user. Therefore, TDMA, in the sense used herein, also includes those FDMA situations where adjacent signal timing may produce interference.
Even with the use of SRN pulse shapers, therefore, a problem exists within a TDMA digital communications system. Such a system incorporates multiple subscriber units for use with a single base station. Each subscriber unit has a transmitter with an independent baud clock. Therefore, guard slots are required between adjacent allocated time slots in order to minimize interference between the subscriber unit transmissions. This represents an inefficient use of time-domain allocations. This inefficiency can severely impede overall system performance. The use of high-speed burst transmissions exaggerates this impediment.
One approach to the minimization of a need for guard slots is to use a long phase-locked loop encompassing both a base station and a subscriber unit to effect the synchronization of a subscriber unit transmit baud clock with a xe2x80x9cstandardxe2x80x9d base station receive baud clock. A marked disadvantage of this approach is that when such a phase-locked loop has a loop filter wide enough to track the phase differences between the baud clocks, the variable-frequency oscillator tends to hunt, producing a high degree of phase jitter. Conversely, when the loop filter is tight enough to prevent excessive carrier phase tracking jitter, it has difficulty tracking the phase noise of the oscillator. These problems are characteristic of phase-locked loops utilizing variable-frequency oscillators.
Accordingly, it is an advantage of the present invention that a method of synchronizing the phases of independent subscriber unit baud clocks with a base station baud clock is provided.
It is another advantage of the present invention that a time division multiple access (TDMA) digital communications system is provided in which, within a base station, a baud clock in each subscriber unit is coherent with a baud clock in that base station.
It is yet another advantage of the present invention that an interpolator is incorporated into a subscriber unit transmitter and configured to adjust a subscriber unit baud clock phase within that subscriber unit so as to cause that subscriber unit baud clock phase to be substantially equal to a base station baud clock phase within that base station.
It is another advantage of the present invention that a long phase-locked loop utilizing a fixed-frequency oscillator is effected to synchronize the phases of a subscriber unit transmit baud clock and a base station receive baud clock.
These and other advantages are realized, in one form, by a method for synchronization of a time division multiple access digital communications system incorporating a base station and a subscriber unit. This method includes the steps of generating a baud clock within the subscriber unit, adjusting the baud clock phase via an interpolator within a transmitter within the subscriber unit, and broadcasting a reverse channel signal having the baud clock as a component thereof via the subscriber unit transmitter,